1. Field of the Invention
The present invention relates to short range communications networks between a reader and a transponder and more particularly to a transponder for operation in a variety of such networks implementing different communications protocols.
2. Description of the Prior Art
Communications systems for the remote identification of objects by electronic means are known in the art. The purpose of such systems may be Automatic Vehicle Identification (AVI) for Commercial Vehicle Operations (CVO) and for Electronic Toll and Traffic Management (ETTM) applications. The objectives of CVO services are to increase productivity of commercial vehicle regulatory agencies and commercial vehicle operators, and to enhance the safety of CVO drivers and vehicles. Examples of CVO services include automated permit and registration acquisition, vehicle performance monitoring, and hazardous materials incident response. ETTM allows drivers to pay highway tolls without stopping, and allows traffic managers to use transponders as probes in high traffic volume areas to facilitate incident detection. These systems provide a two-way communications means between a reader and a transponder (or “tag”). The tag can store information of interest such as identity, fuel level, time of day, cargo ownership and vehicle type, etc. This information may change and be updated as conditions change. Subsequent reading of the tags can keep those persons monitoring the vehicle, cargo or container etc. appraised of the conditions.
Such communication systems use RF signals to communicate between a reader device, such as fixed Roadside Equipment (RSE), and a mobile transponder generally fixed to the object of interest. Some systems permit both “read” and “write” capabilities, permitting a reader to access stored data in the transponder and permitting the transponder to update the data stored therein in response to signals from the reader (eg. to write the time and place of entry onto a toll highway in the transponder useful for calculating tolls based on distance and time-of-day travel).
In operation, the reader sends a RF signal to the transponder. An antenna at the transponder receives the signal from the reader and responds thereto according to one of many developed protocols. The transponder produces a series of signals in conjunction to its identity code, providing the reader with data stored in the transponder which the reader decodes.
Reader-transponder technologies divide into two basic physical modes of operation: active transmission (Active) or modulated backscatter (Backscatter). Active systems utilize a transponder with an active transmitter which responds to interrogations from the reader with an active modulated RF response signal generated by the transponder. In contrast, Backscatter systems utilize a transponder that responds to a continuous wave (CW) RF signal generated by the reader. The tag responds by modulating the continuous wave, electrically switching the tag's antenna from a reflective to an absorptive characteristic according to the tag's modulating signal. While Backscatter systems are typically limited to using amplitude modulation for the response, Active systems may use phase, frequency or amplitude modulation.
As a result of the frequency limitations of Active systems and the fact that such systems cannot employ a multiple number of frequencies within an assigned band, interference between closely located systems is typically controlled by Time Division Multiple Access (TDMA) of the closely located systems. This contrasts to the ability of Backscatter systems to use a number of possible frequencies within an assigned band with less risk of interference between adjacent capture zones. Backscatter systems, therefore, are generally not closely synchronized in time. Both systems however can employ time and frequency multiplexing to control interference.
A natural consequence of these physical differences is that the communications protocol that is most commonly used for either system is also characterized as either Asynchronous or Synchronous. Backscatter systems are typically isolated primarily in frequency instead of time, hence tolerating communication lengths that are uncontrolled and operating asynchronously. Active systems are primarily isolated in time instead of frequency, hence operating in synchronous mode with tightly controlled packet lengths.
Among the developed communications protocols are:    1) various public TDMA protocols for Wide Area or Lane Based operations (See too, PS111-98 Standard Provisional Specification for Dedicated Short Range Communication (DSRC) Physical Layer Using Microwave in the 902 to 938 MHz Band, AMERICAN SOCIETY FOR TESTING AND MATERIALS, ASTM Subcommittee E17.51 on Dedicated Short Range Communication, West Conshohocken, Pa.);    2) State of California Code of Regulation (CALTRAN) Title 21 (T21) protocol (eg. http://www.dot.ca.gov/hq/traffops/elecsys/title21/docs/t21updat.htm); and    3) proprietary IAG (northeastern Interagency Group (IAG) members (NY, NJ, PA, DE)) protocols. See for example, U.S. Pat. No. 4,870,419 of Baldwin et al. issued Sep. 26, 1989 entitled, “Electronic Identification System”; U.S. Pat. No. 5,132,687 of Baldwin et al. issued Jul. 21, 1992 entitled, “Electronic Identification System”; U.S. Pat. No. 5,164,732 of Brockelsby et al. issued Nov. 17, 1992 entitled, “Moving Vehicle Identification System with High Gain Antenna”; and U.S. Pat. No. 5,196,846 of Brockelsby et al. issued Mar. 23, 1993 entitled, “Moving Vehicle Identification System”.
U.S. Pat. No. 5,425,032, of Shloss et. al, Jun. 13, 1995, entitled “TDMA Network and Protocol For Reader-Transponder Communications and Method” also discloses a TDMA protocol.
For TDMA protocols, communications are initiated by a first RF signal transmitted by a reader to a transponder at a defined frequency. In a quiescent mode, the transponder monitors the frequency for incident RF energy of about −30 dBm. The transponder receives and decodes the signal to determine if the signal encodes Manchester Data. Thereafter, the transponder looks for a Frame Control Message (FCM) within the signal received according to the protocol. Once a FCM is determined, the tag moves to an active mode and engages in TDMA protocol communications.
For the T21 protocol, in a quiescent mode, the transponder monitors the defined frequency (eg. 915±13 MHz) for a RF wake up signal according to the protocol (eg. 33 microseconds of unmodulated RF) at a minimum RF level. RF presence of at least about −22 dBm may indicated T21 signals are present. Thereafter, the transponder awakes and actively searches for a T21 Interrogation or polling message and responds appropriately as a passive Backscatter transponder.
For IAG protocols, the transponder sniffs the defined RF band for an IAG trigger signal and wakes up to engage as an active IAG transponder. IAG trigger signals are similar to T21 triggers but differ in length and are not followed by a T21 polling message. In IAG communications, the transponder responds promptly upon receipt of a wake signal, without waiting for a polling message. The RF level required to initiate IAG operation is higher than the RF level to engage T21 activity, which is in turn higher than the RF level to initiate TDMA activity.
Comprehensive standards governing the communications between the transponder and reader, and the message sets on the transponder, do not exist. Therefore, interoperability does not exist between the equipment of different manufacturers. Interoperability, in this case, is the ability of a roadside reading or interrogation device of one manufacturer to meaningfully process the data from any given transponder mounted in a vehicle. The communications industry has been unable to agree upon a path for standardization.
Vehicles, for example, which traverse large geographic areas may be required to respond to a multitude of AVI implementations for electronic toll collection or other commercial vehicle operations purposes and can only do so by selecting the transponder appropriate to the jurisdiction and mounting the transponder to the vehicle. As the vehicle moves from one jurisdiction to another, the operator is required to select from a supply of transponders, mount the appropriate one and shield any others from being inadvertently activated and possibly interfering with the transmissions.
A dual protocol transponder implementing both IAG and TDMA Wide Area protocols (sold under the trademark Fusion of Mark IV Industries) is known in the art. This transponder provides only two synchronous and actively transmitted protocols. The transponder combines the operation of two separate transponders implementing the TDMA and IAG protocols respectively by employing separate protocol detectors and a single communications controller operable according to the detected protocol. For TDMA operation, when in a quiescent mode not engaged in communication with a reader according to a selected protocol, the transponder periodically wakes up and looks for (i.e. samples) RF above a TDMA baseline signal strength and within a designated frequency range. If such an RF signal is detected, the transponder examines the signal to see if it is a TDMA protocol signal. If the detector finds an expected Frame Control Message encoded in the signal, the transponder enters TDMA communications with the reader and otherwise continues monitoring for appropriate signals. Employing a separate detector in parallel for IAG protocol detection, the transponder continuously monitors for RF signals above an IAG baseline. If such a signal is detected, the transponder determines if the signal is an IAG trigger and enters IAG protocol communications appropriately.
Such a transponder has limited functionality in that it does not provide for both active and backscatter communications ability. Moreover, in employing parallel detection strategies with separate detection hardware, the transponder is not efficiently scalable to increased protocol implementation.
It is also impractical to install multiple RSE readers implementing different protocols in an attempt to read different transponders implementing different protocols. Each of the reader protocols is designed to utilize the available time efficiently and makes no allowance for sharing the RF spectrum with incompatible RF protocols that could result in RF interference at the transponder or at the reader. Any such attempt can only operate with the penalty of considerable degradation.
Typically, each AVI system uses different identification means for the transponder. In the absence of compatible identification, the opportunity for general mobility of a transponder is further restricted.
It is desirable that a single transponder be capable of responding to a plurality of communications network protocols in a manner that overcomes the limitations of the prior art.